In the United States, where there is a broad range of climate zones, and an abundance of rain in many areas, there is typically an adequate supply of fresh drinking water available in most regions of the country. Even in areas where water supplies are scarce, efforts have been made to transport water from where it is available, to where it is needed. For example, a significant amount of water is currently being transported from the Colorado River, via the California Aqueduct, to heavily populated, but dry, regions of Southern California, so that sufficient water will be available, not only for drinking purposes, but also for agriculture and irrigation. Other means of supplying and transporting water, such as through a network of utilities and pipelines, including those from lakes, reservoirs, rivers, glaciers, etc., are also in existence.
Nevertheless, there are many geographical areas where it is not as convenient or possible to transport water to where it is needed, on a cost effective basis. These areas include far away regions in the mountains, faraway rural areas, and islands in the ocean, where it might be cost prohibitive to install long pipelines or aqueducts to transport water to those areas. There is also a need for fresh drinking water on board ships, such as those that go for extended periods of time, i.e., cruise lines, ocean liners, cargo ships, Navy fleet ships, etc. There are also other countries, such as where the climate is drier, or where the population is much larger than the availability of fresh drinking water can support. Ironically, many of these areas are located close to the ocean, or other salt water bodies, where seawater is available, but where the amount of fresh drinking water available may be insufficient to meet the demand.
Accordingly, desalination systems and methods to produce fresh drinking water from seawater have been developed in the past. The key to any desalination system is the ability to separate the contaminants, including salt and other impurities, from the base water, which, in turn, can produce fresh drinking water.
For purposes of simplicity, the term “seawater” will be used from this point on to refer to any contaminated water that needs to be purified, whether it is actually water from the sea, or brackish water, or any other water from any other source.
At least three different types of desalination systems are currently in use today, to varying degrees of success, which will now be discussed.
Thermal Method: A thermal method is one that uses heat or other means to convert the seawater into a water vapor, such as by boiling, leaving behind the contaminants in the residual base water. For example, one type of thermal method commonly used is called distillation, where seawater is progressively heated in subsequent vessels at lower pressures to produce a purified water vapor.
The thermal method called Multistage Flash Distillation is the most common desalination method currently being used world-wide. It involves heating the seawater to a high temperature and passing it through a series of vessels having decreasing pressures to produce water vapor. The heated water is passed to another vessel known as a “stage”, where the surrounding pressure is lower than in the heater. The sudden introduction of solute and water into a lower pressure “stage” causes the solution to boil so rapidly that it flashed into steam (water vapor). The water vapor, by its very nature, consists of purified fresh water without the solutes, which can then be condensed, collected and used for drinking water. A similar process called Multi-Effect Distillation uses the same principles as the Multistage Flash Distillation, except that this method operates at lower temperatures. At lower pressure there is a lower temperature required to vaporize a given weight of water. There is also another distillation method called Vapor Compression Distillation which is generally used in combination with other processes, where the heat comes from the compression of vapor, rather than direct exchange of heat.
While these methods are highly successful and effective in being able to remove contaminants from the seawater, one of the most significant disadvantages of these methods is the high cost of operation, including the high cost of the energy needed to heat the seawater, and/or reduce the pressure, to boil it. Because of these costs, these methods are often used only when fresh drinking water is not available, or could not be made available at a lower cost.
Membrane Processes: A membrane process is one that uses a relatively thin permeable layer of material that allows either water or salt to pass through, which helps induce a separation between two differing concentrations. On one side of the membrane is typically located the water with the contaminants, and on the other side of the membrane is typically located the water that has been purified.
One of the most common types of membrane processes is reverse osmosis, which is a pressure driven process which forces saline water through a membrane, leaving salts behind on the other side. While reverse osmosis has been shown to be economical in terms of its relatively low energy consumption rate, the process typically has a significantly higher upfront investment cost compared to most thermal methods. There is also a relatively high cost associated with the replacement cost of membrane material, due to limited membrane life.
Another common membrane process is called Electrodialysis, which is a voltage driven process that uses an electric potential to move salts selectively through a membrane, leaving fresh water behind. This process has some of the same drawbacks as reverse osmosis, in that the upfront capital costs can be prohibitive.
Freeze Crystallization: Freeze crystallization is a process which takes advantage of the freezing process and the phase diagram of seawater to produce fresh drinking water. Essentially, seawater is subjected to cooling temperatures, such as via a refrigerant, which causes freezing to occur, wherein the freezing is used to help form solid ice crystals made from pure water, which can then be separated from the salt contaminants contained in the residual base water. This process and its potential has been investigated due to its higher efficiencies when compared to reverse osmosis, but the freeze process for desalination purposes has never been implemented successfully on a large scale.
The freeze crystallization process is different from the other processes, and typically involves the formation of ice crystals by freezing seawater, wherein the pure water ice crystals can be separated out of the base water. During the process, when pure ice crystals are formed by exposure to freezing temperatures, they are usually allowed to traverse to the top of a tank, under action of buoyancy, while the impurities are allowed to separate and sink by reason of a higher density. This way, it is possible to separate the pure ice water from the impurities, such as the salty brine and other minerals, wherein fresh drinking water can be produced.
In addition, the formation of ice crystals, in this respect, can be achieved in two different ways: 1) by direct cooling, and 2) indirect cooling.
Direct cooling involves using an inert cooling fluid or refrigerant that is physically injected into the seawater, i.e., bubbled through it, which causes the seawater to vaporize at the desired temperature. This is the result of the refrigerant heat of evaporation being drawn from the solution, which causes the seawater to cool down to the eutectic temperature. Although the intermixing of the refrigerant with the seawater makes this method efficient, an important drawback is that the refrigerant ends up getting disadvantageously intermixed with the ice. That is, ideally, the process would form distinct particles of pure water ice, but in this cooling method, some of the refrigerant, which is present throughout the equipment, ends up being trapped within the ice particles, and therefore, fresh drinking water, free of the refrigerant, cannot be produced with purity. Consequently, the direct cooling method has not been used in connection with the production of fresh drinking water.
A more common freezing desalination process is called indirect cooling. In this process, the refrigerant is not injected directly into the seawater, but rather, is introduced into a series of pipes or jackets that run through the seawater. The pipes and jackets are typically made of a material, like aluminum, with good heat conducting properties, and help keep the refrigerant and seawater separated during the process. The refrigerant preferably only passes through the inside of the pipes and jackets; the seawater, on the other hand, is only exposed to the exterior of the pipes and jackets, and not the refrigerant itself. This way, the seawater is cooled by direct contact with the pipes and jackets, and not direct contact with the refrigerant. This avoids the problems associated with the direct cooling method, although using pipes and jackets introduces an added level of resistance to the transfer of heat from the seawater to the refrigerant, which makes this method less efficient.
Once the pipes and jackets are cooled, and seawater is passed over them, ice crystals are formed on the exterior surface, and therefore, it becomes necessary to remove the ice from the surface physically, such as by using a scraper. Once the ice is physically removed, the lower density ice crystals that are released tend to float to the top of the higher density brine solution. This way, the ice crystals can be removed from the top, and washed clean of impurities, and then allowed to thaw and melt, wherein they produce purified drinking water. Thereafter, the salty brine mixture can be disposed.
The total surface area of contact that is needed for indirect cooling and the heat transfer coefficient are the key parameters of this process. These parameters relate to the effectiveness of the refrigerant in being able to cool and freeze the seawater solution around the refrigerant-containing pipe or jacket.
An example of an indirect cooling method in use currently is called a Scraped Surface Crystallizer, which consists of a cooled cylinder (evaporator) with a knife or scraper arrangement that is able to scrape ice crystals off of the cooled surface. Either the scraper moves over the cooled surface, or the cooled surface is moved across the scraper, to enable the ice to be scraped and removed. In either case, to avoid friction and damage to the surfaces, the system is typically designed so that there is a clearance or gap between the surfaces, which ultimately leaves behind a layer of ice on the cooling surface. This additional layer of ice on the cooling surface causes the entire system to be inefficient in its ability to transfer heat away from the cooling surface, to allow more ice to be formed on the surface, and causes a sharp decrease in heat flow through the cylinder, and a drop in its capacity. There is also the adhesion force that exists between the scraper and the cooling surface which requires the expenditure of energy to overcome, i.e., extra energy is needed to scrape the ice to remove it successfully, and this must be accomplished constantly over an extended period of time.
There have also been studies done involving freezing wastewater to determine how ice crystals form within a droplet. In studies by Dr. Wa Gao, discussed in her thesis entitled “Partial Freezing by Spraying as a Treatment Alternative of Selected Industrial Wastes,” the effect of freezing temperatures on a single droplet of wastewater was studied, wherein a single droplet (at below freezing temperatures of fresh water of about minus 5 degrees C.), was exposed to an updraft of subfreezing air temperatures, i.e., between minus 5.5 degrees C. to minus 17.7 degrees C., in a chilled air vertical wind tunnel. The test was performed repeatedly for the same droplet size and droplet condition to obtain statistical information. She observed the freezing of each droplet began at the bottom edge of the droplet, and then enveloped the outer surface area of the droplet in 0.23 seconds. The wastewater froze inwardly as the ice shell thickened. The complete 2,800 micron droplet froze completely in a mean period of time, i.e., 7 seconds. In all cases the freezing was complete in 20 seconds. The solid ice portion of the droplet was formed with pure water, and the remaining liquid brine around it consisted of the concentrated wastewater. The ice spheres fragmented during the freezing process as the interior liquid brine was squeezed to the outside because of internal stresses in the ice as it squeezed the incompressible liquid. In another study, wastewater was sprayed outdoors in cold arctic winter weather to produce a large mound of ice particles directly on the ground. Although initial freezing took place while the droplets were in flight, additional freezing and separation occurred after the droplets landed on the ground. Once the mound was formed, it could then be melted during the spring, and used for irrigation purposes.
Because of the drawbacks of the existing desalination methods and systems discussed above, there is a need for a highly efficient and cost effective desalination method that allows fresh drinking water to be produced from seawater.